Drugs dispensed from MDIs usually consist of very finely divided particles, typically in the 1 to 8 micron range. The medication particles are suspended in liquid propellant such as Freon or the like which is under pressure in the MDI canister. Upon actuation, a metered dose of the drug and propellant is ejected through the outlet tube of the canister and, in the prior art, out through one or at most two ports or orifices that are aimed in the longitudinal direction of the air stream to the patient. See, for example, U.S. Pat. No. 5,012,803 for a description of a single orifice nozzle and U.S. Pat. No. 5,474,058 for a two orifice nozzle construction.
As the mixture of drug and propellant is ejected out of a nozzle, it is accelerated to a high velocity so that shear forces with the nearly stationary ambient air cause the mixture to break up into many small, rapidly evaporating droplets, each of which contains hundreds to thousands of drug particles. The exit ports of the prior art dispensers typically are about 0.5 mm in diameter for single-orifice dispensers and 0.3 mm in diameter for dual-orifice dispensers.
The plume of propellant and agglomerated drug that exits a round orifice nozzle travels several tens of millimeters before the propellant can gain enough heat from the surrounding air to evaporate. The evaporation of the propellant is a phase transition that requires the input of heat to change the propellant from liquid to vapor. The rate at which heat can be transferred from the air to the propellant droplets is the limiting mechanism for their evaporation.
A major problem with MDI ventilator dispensers is that the expanding plume, consisting of unevaporated droplets containing drug particles, impinges upon the walls of the ventilator circuit and remains there, forever lost to the patient. One method to minimize this loss of drug is to add a large diameter spacer to the circuit in which the plume may expand, as described, for example, in U.S. Pat. Nos. 5,012,803; 4,484,577; 4,790,305; 4,938,210 and 5,178,138.
There are, however, several disadvantages associated with use of large-volume spacers. Their weight tends to pull on the ventilator tubing which is inserted into the patient's trachea which may cause the patient considerable physical discomfort. They also collect contaminated fluid. The volume of the spacer is also a hindrance to optimal air flow to the patient because the spacer adds needless volume to the circuit. And, in collapsible versions of a large-volume spacer, such as described in U.S. Pat. No. 4,938,210 the spacer may be difficult to open once it has been collapsed.
An advantage exists, therefore, for an aerosolized medication delivery device that would enable rapid expansion and evaporation of the medication's pressurized propellant, thereby resulting in a compact, lightweight device which would efficiently deliver medication yet not cause the patient undue discomfort.